The present invention relates to a color image forming apparatus wherein a plurality of developing units each containing a differently colored developer are provided around the peripheral portion of an image-carrier and a plurality of differently colored images developed by aforesaid developing units are superposed on the image carrier and then transferred onto an image-transfer material to be a color image.
Many methods and apparatuses for obtaining a color image through an electrophotographic method have been proposed. In the method disclosed in Japanese Patent Publication Open to Public Inspection No. 100770/1986 (hereinafter referred to as Japanese Patent O.P.I. Publication), for example, formation of latent image and development thereof in the number of times corresponding to the number of colors separated from an image on an original are made on a photoreceptor drum that is an image-carrier, and an image of each color is transferred onto a transfer drum for each development to form thereon a multicolor image which is transferred onto a recording paper to be a color image. An apparatus based on this method requires a transfer drum having the size to cover by its circumferential surface an image equivalent to one entire sheet in size, in addition to a photoreceptor drum. Thus, it is impossible to avoid the apparatus being large and complicated. Further, in the method disclosed in Japanese Patent O.P.I. Publication No. 149972/1986, for example, formation of latent image and development thereof in the number of times corresponding to the number of colors separated from an image on an original are made on a photoreceptor drum and each developed image is transferred, for each development, onto a transfer material to form a multicolor copy. In this method, it is difficult to superpose images each being in different color accurately in position, thus, it is impossible to obtain a color copy with an excellent quality.
There further is a method wherein formation of latent image and development thereof by means of color toner are repeated for the number of times corresponding to the number of colors separated from an image on an original, and after color toner images are superposed on a photoreceptor drum, they are transferred to become a color image. The basic processes of the multicolor image forming mentioned above are disclosed in Japanese Patent O.P.I. Publication Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985 and 158475/1985.
In the multicolor image forming apparatus for forming a color image through superposition mentioned above, a plurality of developing units each containing a differently colored toner are provided around a peripheral portion of a photoreceptor drum, and the photoreceptor drum is generally caused to make plural turns, thereby to cause a latent image thereon to be developed to be a color image.
The photoreceptor drum is generally caused to make plural turns, and during that period, toner images each having a different color are superposed on a photoreceptor drum, then they are transferred onto a transfer material which, after completion of transfer thereonto, is subjectedto fixing and is ejected to the outside of an apparatus. The photoreceptor drum from which the toner images have been transferred, on the other hand, is cleaned by a cleaning means located at the downstream side from the transfer position for the next image forming process.
As a cleaning means, a cleaning blade or a rotating fur brush is used, and it is kept away from the photoreceptor drum during the period of image forming, and after the transferring of images is completed, it is caused to be in contact with the photoreceptor drum and scrapes off residual toners thereon to clean the circumferential surface of the photoreceptor drum. Waste toners scraped off are collected and conveyed to a waste toner box. However, it is impossible to convey all waste toners collected to the waste toner box, and accordingly, a part of waste toners stay, for example, on the surface of an edge portion or the like at the tip of the cleaning blade. As shown in FIG. 4-A, such toners staying on the tip of the edge portion sometimes drop on a drum surface while cleaning blade 91 is retreating or leaving the circumferential surface of photoreceptor drum 10 after being released from its pressure contact on photoreceptor drum 10, and stay again on the photoreceptor drum surface.
FIG. 4-B shows a circumferential surface of the photoreceptor drum developed in its rotation direction shown with arrow A. Aforesaid toner staying portion (a hatched portion) is located successively at the upstream side against the point B where the cleaning blade starts retreating. However, since most of such staying toners stay in a relatively narrow area, an amount of staying toners which are transferred to a recording paper is relatively small when the recording paper that is an image-transfer material is fed to the ordinary position against an image for transferring thereof.
However, when the recording paper is fed slightly earlier than the image for the purpose of providing `a binding margin` and others, aforesaid toner staying portion interferes in the recording paper zone, resulting in the margin (binding margin) soiled with toners.
As stated above, toners staying on a cleaning means stick to the portion in the vicinity of the position where toners scraped from the image carrier stay, but a part of them move to other potential gap generated by other process and stay there.
When toners staying on a potential gap portion on a photoreceptor drum are developed, such toners cause an image soil problem for an image formed in the next cycle. This phenomenon causes a problem in a color image forming apparatus wherein a color image is formed on a photoreceptor drum through a plurality of rotations of the photoreceptor drum. An object of the present invention is to provide a color image forming apparatus which offers a clean image having no image soil.